3. Using TFCompanion. 3.1 Filmstack. Introduction Layer types and properties

Size: px

Start display at page:

Download "3. Using TFCompanion. 3.1 Filmstack. Introduction Layer types and properties"

Silvester Austin
5 years ago
Views:

1 3. Using TFCompanion. 3.1 Filmstack. Introduction. Filmstack is an optical model of the sample that is measured - it consists of a substrate, collection of layers and an ambient. There is no limitation on the number of layers; they can be homogeneous or graded with specified profile. Substrate and ambient are infinite (do not have thickness), all layers have finite thickness. In the cases, when physical substrate is transparent and one need to measure transmission or backside reflectance need to be taken into account physical substrate need to be represented as a layer in the filmstack model (substrate will be Air or other material if the sample is immersed). Thick transparent layers are supported software will detect automatically if the thickness exceeds typical coherency length of light and give an option to the layer to incoherent type. As an example, glass slide with roughened backside is a substrate (no reflection from the back side); however, the same slide with the polished backside is an incoherent type layer. The key factor is whether reflectance from the backside need to be taken into account (measured by detector) or it does not. Any element of the filmstack can be edited (ambient, substrate, layers). One can create filmstack (add/insert/remove layers). One can also modify parameters of the existing filmstack (e.g. change material of the layer or thickness, etc.). By default all layers and materials in filmstack are independent, however, one can explicitly link layers and /or materials Layer types and properties Layer is a convenient optical model abstraction of a physical film. Layer always has a finite thickness and material associated with it. TFCompanion supports two types of the layers: Homogeneous layer. Optical properties have uniform profile along the thickness (z) coordinate. Graded layer. Optical properties have specified profile along the thickness (z) coordinate. This profile is represented by dividing layer on sub-layers (grades) with specified optical properties.(see section 3.5) Layer properties. Independent and linked layers. By default all layers and materials in filmstack are independent, however, one can explicitly link layers and /or materials. Linking layers and materials is inter-related and one needs to exercise care when mixing them together. Linking of layers automatically links corresponding materials (one cannot unlink materials of the linked layers), i.e. linking of materials supersedes linking of layers. For example, linking two materials does not link corresponding layers but linking two layers does link respective materials. Linking two objects (layers or materials) means that all the parameters (properties) of these objects are linked; consequently only two equal layers can be linked (must have the same thickness and material, in case of the graded layer also the same profile, etc.). When linking layers, one needs to select a node (master layer) and link other layers to this master (there can be several nodes each with its dependent layers). The node is the layer that is visible to calculation routine and displayed calculated parameters have this layer designation. Material of the node (master) layer becomes itself a node of the related linked

2 materials. It is possible to link layers and link another material without linking corresponding layer. One example can be linking two oxide layer and adding oxide material of the third layer to the link (e.g. material can be a part of the EMA). The link map of layers and materials is displayed in the Links panel (Filmstack panel in the Main window). Linking and unlinking can be also done at that place. Surface Roughness(random) Effect of random surface roughness is manifested differently depending on the measured parameter. Reflectance and transmittance measurements are affected directly and the effect can be estimated using Kirchoff scalar model. Roughness value (rms in Angstroms) can be selected in Calculation dialog. At the same time, ellipsometry is affected by roughness in a different way. The best way to model roughness effect on ellipsometry measurement is to represent it as a separate layer, perhaps EMA of material+void or oxide, etc. In addition, surface roughness can cause some depolarization and increase the noisiness of the data. Note. Using rms roughness setting in Calculation dialog will have effect only on reflectance /transmittance data and no effect on ellipsometry data Incoherent layer There is an upper limit on the thickness of the layer that can be measured using reflectance/ellipsometry methods at specific wavelength. When thickness of the layer exceeds coherence length of the light used for measurement the phase is undetermined, consequently there is no interference between front and backside reflected light. This thickness depends on the wavelength of light and its type (laser or lamp). In most cases, the thickness of few hundred μm makes layer incoherent in visible spectral range. By default, all layers are coherent. When thickness is set (Parameters table in Filmstack panel main Window) the coherence length is estimated and if the layer is thick enough user is prompted to select incoherent layer type. If the thickness is decrease and layer is incoherent user is prompted to set layer coherent Loading existing filmstack Filmstack objects are stored in a database. Filmstacks are self-consistent, i.e. they are not relaying on external references/links. This mean that it contains copies of all the materials that it uses there is no link between materials database and filmstacks database. There are several ways to load filmstack: Loading from the database: Filmstacks database (along with other databases) is displayed in the left-sidebar of the main dialog. One needs to select the record, right-mouse click and select Set Filmstack from the pop-up menu. Loading from the cache: Filmstacks can be cached in memory at the various stages of a project. To load filmstack from the cache one need to select Tools/History (from the main menu), select the filmstack record and click Enter. Indirect loading. o o Filmstack can be loaded as a part of a Calculation Recipe (calculation recipe is stored in a separate database and may contains a reference to a filmstack record.) Filmstack can be loaded as a part of a Measurement Recipe

3 o Default filmstack loading as start-up of the application. User can select a filmstack that is loaded at application start-up in configuration dialog (Configure/Software Configuration select Sources tab, select browse in Pre-loaded filmstack panel and select a filmstack from the database and save a new configuration) Creating Filmstack. There are two ways to create/edit filmstack: Directly in the main dialog by either using Filmstack menu (Layer Add, Layer Insert, Layer Delete) or toolbar options or materials database in the sidebar. This method is recommended for small changes of the filmstack (e.g. changing (adding, deleting, inserting one layer). Using Filmstack dialog page. Select Filmstack/Edit Filmstack from the main menu. This method is recommended in case of significant changes of the filmstack (e.g. creating multilayer filmstack) Creating Filmstack Directly Adding a Layer. Method 1: Select Layer Add from the Filmstack menu (main menu) or Select icon in the Toolbar. The list of materials available in the current material library will appear in a separate window. The name of the library, its location and creation date are shown in the status bar at the bottom of the window. Select (click on) the material you need. The name of the material and comments appear in the respective text fields. At this point you can enter selected material (button enter), view/edit material (button View/Edit) or cancel (button cancel). Select Enter. The dialog with a text field to input Thickness will popup. Thickness must be entered in Angstroms. Click O.K. button and the new layer will be added to the current filmstack. Click Cancel button if you changed your mind the layer will not be added to filmstack. Select View/Edit. Material dialog is displayed. This option allows reviewing and/or editing of Material properties (any changes to the material need to be saved to the Database before closing Material dialog, otherwise they will be lost). Method 2: Use the left sidebar (select Materials tab to display the materials record ) to select the material; right-click mouse to select (Add Layer or Review/Edit option as above) Deleting a Layer. Select Layer Delete from Filmstack dropdown menu or select icon from the Toolbar. If there is only one layer - it is removed from the filmstack. If there is more than one layer a dialog with a list of layers is displayed. Layers are numbered from the substrate, i.e. if there are 2 layers the bottom layer is #1 the top layer is #2. Select the

4 layer you would like to delete and click O.K. button. Selected layer is removed from the filmstack Inserting a Layer. Method 1: Select Insert Layer from the Filmstack dropdown menu or select icon from the Toolbar. The list of materials available in the current material library will appear in a separate window. All the functionality is the same as for Add Layer. After material is selected and the thickness is set, a dialog will appear with a list of all available layer # s where a layer can be inserted. These numbers correspond to the location of the inserted layer. For example, if filmstack has two layers Si3N4/Sio2/Si-substrate then the new layer (poly) can be inserted in position #1 (Si3N4/Sio2/poly/Si substrate) or position#2 (Si3N4/poly/Sio2/Si substrate). Note. If there is one layer, dialog will not appear and the new layer will be inserted to the only available position#1 (previous layer will move up correspondingly). Method 2: Use the left sidebar (select Materials tab to display the materials record) to select the material; right-click mouse to select Insert Layer from the popup menu (the same action as in Method 1 above) Using FilmstackCreator Dialog Using FilmstackCreator allows one to quickly create or modify Filmstack. Select Filmstack/ Filmstack Edit from the dropdown Filmstack menu or select icon from the Toolbar FilmstackCreator dialog will appear. Current filmstack is listed in the Filmstack List There are two ways of editing creating filmstack in the Filmstack creator: Quick Filmstack -- for advanced users; Regular Filmstack for novices. RegularFilmstack and QuickFilmstack are interchangeable, i.e. filmstack created using Quick Filmstack Creator appears in the RegularFilmstack window as well as in the QuickFilmstack window. Either way, after filmstack is created click OK button, program returns to the main window and filmstack is updated with new data. Selecting OK button commit all changes to the database, selecting Cancel button rollback all the changes. If new or modified filmstack is not saved to the database (Save button) it will be available only during current session Quick Filmstack Quick Filmstack is designed for advanced users who know the available materials and their exact names. All filmstack can be typed in accordance with the common notation used in the printed materials. The signature is: <top layer></><next layer></> </><bottom layer></>< Substrate material s name >. Each layer has a following signature: <Thickness in Angstroms><material s name> Example: 100Gaas.mat/100Sio2.mat/Si.mat After typing this String, click Enter. The string is parsed and filmstack is created materials appear in the Filmstack window. If one of the materials is not recognized as a qualified name -- the error box appears and Quick Filmstack string is cleared. If the

thickness value is missing or not recognized dialog with the input field appears prompting to input thickness (in Angstroms) for particular layer.

5 thickness value is missing or not recognized dialog with the input field appears prompting to input thickness (in Angstroms) for particular layer. The same way Quick Filmstack can be used to modify/edit existing filmstack. When the FilmStack Creator dialog appears, Quick Filmstack string corresponding to the current filmstack is set in the window. The user can manually modify materials, thicknesses, and insert or add new layers. Fig FilmStack Creator Dialog Regular Filmstack RegularFilmstack is designed for novices, who may not know the exact material names. All materials available in the database are listed in the left sidebar library. The Current filmstack is listed in the Filmstack List window and in the QuickFilmstack window. User has full control over filmstack, he can add, insert, remove or change any layer or substrate, set or change the thickness of any layer. Add Layer Select material in the list and use Add button.

6 Insert Layer Select material in the list, select the place to insert in Filmstack list (new layer will be inserted under select layer) and use Insert button. Removing Layer Select the layer you want to remove in the Filmstack List use Remove button and the layer is removed from the filmstack. Changing Layer Select material in the list, select the layer you want to change/substitute, use Change button new material will be set to selected layer Replicate layers Select the layers you want to replicate, use Replicate button; you will be prompted to select the number of replication. Selected group of layers will be replicated specified number of times (added on top of select group) Set thickness Select the layer in the Filmstack List selected layer and its current thickness is displayed. Edit the data in the Thickness text field and use Set button to update the thickness. Review layer Double-click on the layer in the Filmstack List the Layer dialog with all the information about the layer is displayed. You can edit Layer using - all the updates are carried back to filmstack after Layer dialog is closed Caching Filmstacks and Filmstack History While working on the project user may change the original (starting filmstack). Each new change to the filmstack is done directly on the object not on the copy; as result-previous versions of the filmstack, unless saved to the database, are lost and cannot be recovered. User has two options either save filmstacks to database or save it to cache (memory). The difference between these two options is that database will allow using filmstack during future session for all the projects; caching saving filmstack to a current project and allows returning to this filmstack only during the current session (user can also save the Project to the database and reload it during next session). It may be useful to save intermediate filmstacks to cache and, perhaps, later save some selected filmstacks to persistent storage. Cached filmstacks can be reviewed during the session and user can return to them at any point. To cache current filmstack use Filmstack/Cache Filmstack menu item (or use toolbar icon). To review cached filmstacks use Tools/History menu item (or use toolbar icon). The FilmstackHistoryDialog appears and displays all currently cached filmstacks. User can review filmstacks and make any of filmstacks current.

Fig. 3.1.6 Filmstack History dialog. Section 3.

7 Fig Filmstack History dialog. Section 3.2 Parameters Calculation Optical measurements are indirect and filmstack parameters are inferred from the best fit of the measured data to the data generated (simulated) from the selected optical model of the filmstack. This process of finding parameters of the filmstack based on the measured data (target) is mathematically known as solution of inverse problem. This problem can be solved analytically only in limited number of simple cases. General solution is achieved by numerical methods of minimizing the difference between measured (target) data and simulated data, using selected optical model of the filmstack. Selected parameters of the filmstack are adjusted until the minimum is achieved. 2 1 N M 1 i, j P meas P calc ( ) w, 2 where index i indicates the type of the parameter (e.g. in the case of ellipsometry measurements i=0,1 with 0 - corresponding to Delta, 1 - corresponding to Psi) index j indicates the point in the data set; meas P is a measured parameter i at point j; calc P is calculated (or generated) parameter i at point j; is stadard deviation of the measurement parameter i at point j; w is weight of parameter i at point j; N is number of measurement parameters*points; M is number of calculated filmstack parameters; 2 is a merit function that is being minimized

8 TFCompanion uses modified Marquardt-Levenberg minimization that gives fast and robust convergence. However, this method is sensitive to the starting values of the minimization (that is initial values of the filmstack). If a starting value of a parameter is relatively far away from the correct value, minimization can be trapped in the local minima, and global minimum real solution will not be found. It is especially an issue when the films in the filmstack are thick thickness more than one cycle for the wavelength of measurement. (The optical cycle is a thickness of a film corresponding 360 deg. change of a phase. Actually, many commercial ellipsometers are measuring phase (i.e. DELTA) in the degree range, so practical cycle thickness is twice thinner in this case). There are several methods that can be used to improve chances of better convergence and avoid the local minima: Check before starting calculation. Use SimulateFit option in the main screen to check how close (or far away) is your initial filmstack model to the correct result. SimulateFit generates the data set using your filmstack model, the data set is matching the measured data (parameters, wavelength, angle ranges, etc.); it does NOT do minimization. The source of discrepancy in normally easy to see and fix in the simple filmstacks. Use Order Search (sometimes called Grid search) if thickness of some layers is not known reasonably accurate. Order Search can be done on one or several layers. In a nutshell, the idea of the Order Search is to perform minimization with different starting values, compare the Goodness-of Fit (GOF) of the results and select the result with the best GOF as a solution. Order Search can be set in the Layer dialog. Details of the OrderSearch implementation are discussed below. Use constraints on the calculated parameters. Constraints allow limiting permitted range of the calculated parameters. In general, minimization program works best without constraints or with very loose constraints. However, there are situation, e.g. correlation between calculation parameters is strong, when the use of constraints is necessary. Note. Do not set constraints too tight In the majority of practical cases calculation can be done quickly and the results are reliable and unambiguous. However, in the complex cases or during the development of the model one may need to be patient. Calculation dialog provides full control over minimization process and all available information and tools to make the process as convenient as possible. The following steps need to be performed in order to do the calculation: 1. Create the filmstack 2. Load the measurements 3. Select calculated parameters and set calculation conditions (constraints, order search, etc.), if necessary. 4. Calculate Note. In the integrated version of TFCompanion several steps are encapsulated in the measurement recipe. In this case, user loads measurement recipe and select Measure Create a Filmstack Creating of the Filmstack is discussed in the previous section 3.1 Filmstack

9 After filmstack is created, its graphical representation as a stack of layers (buttons) is displayed on the main screen (Filmstack tab in Filmstack panel). More details about filmstack are displayed in the Parameters and Records tabs of the Filmstack panel. Clicking on the layer or substrate brings Layer Dialog with all details of material and layer parameters Select parameters that should be calculated or/ and displayed Any filmstack parameter can be selected as calculated, displayed or both: Calculated parameters are, well, calculated their value is adjusted during the fitting process. Displayed parameters are not adjusted directly during fitting process but are displayed at the end of the calculation. For example, photoresist layer dispersion can be represented using Cauchy model. Calculated parameters will be Cauchy coefficients. However, the user is interested in n,k values at specific wavelength. N,k at specific wavelength can be selected as a displayed parameters. At the end of the calculations, the program will calculate selected n, k values using Cauchy coefficients and display them. In most cases parameters selection can be done in the Parameters tab (Filmstack panel in the main dialog screen): select thickness calculation using Solve check boxes in Filmstack Details table and Solve for panel n, k related parameters. Note. Solve for can be used for selection of direct n, k calculation and for indirect n,k calculation in case of parameterized materials. For example, in case of the Cauchy material n calculation selection (at specified wavelength) will prompt user to select actually calculated Cauchy coefficients (n value at specified wavelength will a displayed parameter. One can select all keyword instead of wavelength in this case all the n,k values matching measured data wavelengths will be calculated, e.g. this can be used to calculate optical properties of a substrate material, etc. Full access to all parameters is available in the Layer Dialog that can be started by clicking on the layer. Parameter can be marked as calculated and/or displayed. Selected parameters are displayed in the Selected Parameters panel (click on Calculated or Displayed tab to view respective parameters) Select Order Search Option (optional) Order Search option can be selected in the Layer Dialog. By default OrderSearch is set to None. There are two OrderSearch option available: Restricted and Standard. Standard OrderSearch automatically selects a span of two cycles: one cycle up and one cycle down from the current thickness. If current thickness is less than a cycle thickness only one cycle (up) is selected. Custom OrderSearch allows custom selection of the Minimum Thickness, Maximum Thickness and Number of Points in the grid Setting constraints (optional) Every calculated filmstack parameter can be constrained, i.e. the range of allowed values can be limited. Constraints can be set in the Calculation Dialog, which displays a table of all the calculated parameters; minimum and maximum constraints values can be

10 edited directly in the table. In addition, layer thickness can be constrained directly in the main screen (Parameters tab in Filmstack panel).

TFCompanion Standard Edition ( Ver ) (build ) User Guide. (Revision #092003) PRODUCTIVITY SOLUTION FOR SEMICONDUCTOR INDUSTRY

TFCompanion Standard Edition ( Ver. 2.5.1) (build 090203) User Guide (Revision #092003) PRODUCTIVITY SOLUTION FOR SEMICONDUCTOR INDUSTRY 1 SEMICONsoft, Inc September, 2003 Important Notice. This manual